<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>上中下</title>
</head>
<style>
    .content{width: 100%;}

    .on {
        background-image: url(../响应式/img/上.png);
        background-repeat: no-repeat;
        margin: 10px;
        background-size: 100% 100%;
        width: 100%;
        height: 100px;
    }

    .in {
        background-image: url(../响应式/img/中.png);
        width: 100%;
        background-repeat: no-repeat;
        margin: 10px;
        background-size: 100% 100%;
        width: 100%;
        height: 500px;
    }

    .under {
        background-image: url(../响应式/img/下.png);
        background-repeat: no-repeat;
        margin: 10px;
        background-size: 100% 100%;
        width: 100%;
        height: 100px;
    }
</style>

<body>
    <div class="content">
    <div class="on"></div>
    <div class="in"></div>
    <div class="under"></div>
</div>
</body>

</html>